A. Significance A1. Infertility. Historically, the quality of life of infertile couples has been greatly diminished by the loss of opportunity to conceive. However, in recent years, novel clinical interventions such as intracytoplasmic sperm injection have dramatically changed the outlook for some couples, particularly those with severe forms of infertility[23]. In parallel with clinical successes, there have also been ground-breaking scientific advances including sequencing of the human genome, derivation of human embryonic stem cell (hESC) lines, and reprogramming of adult human somatic cells to pluripotency[24-28]. Together, these advances now allow us to overcome two historically-insurmountable limitations in studies of human development: the Inaccessibility of early human development to exploration and the genetic-intractability of the genome during development. A2. Need to study human germ cell development. 10-15% of couples are infertile, yet little is known of underlying pathologies in men and women with poor germ cell production. Here, we propose to extend our previous studies in order to permit genetic analysis of human germ cell development and thus, enable novel basic and clinical studies and applications. There are several unique aspects to human germ cell development that merit this investment. First, genes and dosages required for human germ cell development differ from those of mice, including both autosomal and sex chromosomal genes and dosages[6,29-34]. Second, humans are rare among species in that infertility is common, with half of all cases linked to faulty germ cell development[35]. Moreover, pathologies associated with meiotic errors are numerous in humans relative to other species, with errors in meiotic chromosome segregation occurring in as many as 5-30% of human germ cells depending on sex and age[36]. This is in contrast to frequencies of 1/10000 cells in yeast, 1/1000 cells in flies, and 1/100 cells in mice. With recent advances, we now have the ability to incorporate new strategies in order to examine the specifics of human germ cell development. This will allow us to derive full benefit from the wealth of data from model systems such as the fly and the mouse, to begin to understand the complex genetics of human germ cell formation and differentiation. In seeking to understand germ cell biology, we also acknowledge that the ability to contribute to the germ line is a fundamental property that distinguishes pluripotent stem cells. For example, Han and colleagues recently demonstrated that by addition of a 5th factor to the commonly-used 4 factor mixture for reprogramming, ability to contribute to the germ line was significantly increased[37]. This was in spite of the fact that IPSCs derived from 5 factor-reprogramming were indistinguishable from standard iPSCs or mESCs in gene expression and markers of pluripotency. Thus, the research proposed here allows us to address fundamental questions regarding our germ line origins, function, and pathology and lays the groundwork for designing rational therapeutics and diagnostics to inform clinical decisions based on data obtained from model organisms, human genetic studies and direct experimental analysis of human germ cells. It also contributes to the related field of pluripotent stem cell biology and regenerative medicine.